With concerns over environmental issues, resources issues and the like, solar cells have received much attention as a clean energy source. Typical of the solar cells are those which use single crystal silicon, polycrystalline silicon, or amorphous silicon. However, since the conventional silicon solar cells require a decompression (or vacuum) process, the manufacturing cost thereof is high. In addition, for reasons such as the supply of the raw materials of the silicon solar cell being unstable, it is difficult to supply the solar cells at a low price. Thus, there are many problems to be solved to diffuse the solar cells widely.
Furthermore, compound solar cells, such as ones using the Cu—In—Se system (also called the CIS system) have been developed, and have superior features such as having an extremely high photoelectric conversion rate. However, problems such as cost and environmental load also prevent them from being diffused widely.
On the other hand, a dye-sensitized solar cell (referred to as “DSC” hereinafter) has been suggested by a Swiss research group led by Graetzel et al. or the like. Since DSC hardly requires the decompression (or vacuum) process in manufacturing, DSC has received attention as a photoelectric conversion element which can be manufactured with low cost and can obtain excellent photoelectric conversion efficiency (see Non-Patent Document 1).
Generally, a wet type solar cell including DSC has a structure in which an electrolyte is held between a transparent photo electrode letting light enter and a counter electrode composed of a conductive glass substrate.
FIG. 27 is a schematic sectional view illustrating a structure of a conventional wet type solar cell.
This DSC 200 generally includes a first base material 201 on one surface of which a porous semiconductor electrode 203 (also referred to as “dye-sensitized semiconductor electrode” hereinafter) on which a sensitizing dye is supported is formed, a conductive second base material 204 on which a catalyst layer 205 is formed, and an electrolyte layer 206 which is made of gelled electrolyte or the like and is sealed into the space between the first base material 201 and the second base material 204.
As the first base material 201, for example, a plate material having light transparency may be used, and on the surface of the first base material 201 contacting the dye-sensitized semiconductor electrode 203, a transparent conductive film 202 is disposed in order to impart conductivity. The first base material 201, the transparent conductive layer 202, and the dye-sensitized semiconductor electrode 203 constitute a photo electrode (also referred to as “working electrode”) 208.
On the other hand, since the second base material 204 exchanges electric charges with the electrolyte layer, the catalyst layer 205 made of carbon, platinum, and the like is disposed on the surface of the second base material 204 contacting the electrolyte layer 206. The second base material 204 and the catalyst layer 205 constitute a counter electrode 209.
The first base material 201 and the second base material 204 are disposed with a predetermined space in such a manner that the dye-sensitized semiconductor electrode 203 and the catalyst layer 205 oppose each other, and sealants 207 are provided at the periphery of the space between the two base materials. The two base materials 201 and 204 are bonded each other via the sealants 207 to assemble a cell, and an organic electrolyte solution containing redox couples such as iodine/iodide ion is filled between the two electrodes 208 and 209 via an electrolyte inlet 210, thereby forming the electrolyte layer 206 for charge transport.
On such DSC, a sealing operation is carried out before use in order to prevent leakage and volatilization of the electrolyte.
The sealing technologies of the DSC may be roughly classified into the following two groups.
One is a method using resin for the sealing material, in which sealants made of thermoplastic resin are disposed in the periphery of the space between the photo electrode and the counter electrode, the two electrodes are bonded via the sealants by curing the sealants, and an electrolyte is injected thereinto (see Non-Patent Document 2 and Non-Patent Document 4, for example).
Another is a method using glass for the sealing material, in which sealants made of glass with a low melting point are disposed in the periphery of the space between the photo electrode and the counter electrode, the two electrodes are bonded via the sealants by heat-melting the sealants, and then an electrolyte is injected thereinto (see Non-Patent Document 3 and Non-Patent Document 4, for example).
With such sealing technologies, when the sealing material is resin, there are merits in work operations such that the process becomes simple since the sealing material can be sealed at ordinary temperature or a temperature below 140° C., which is the decomposition temperature of a dye. In particular production speed becomes high when hot-melt resin or UV-curable resin is used. However, there is a demerit of inferior durability. On the other hand, when the sealing material is glass, there are demerits in work operations such that the process speed is slower than the case of resin since a temperature above 450° C. is required for glass melting, and that the yield rate is inferior since pinholes and cracks are easily made. However, there is a merit of superior durability.
When DSC is used under a high temperature for a long period of time, air bubbles tend to be generated due to leakage and volatilization of the electrolyte, or due to the change of pressure within the cell. These air bubbles tend to be generated not only in the vicinity of the sealants in the cell but everywhere. At the part where the air bubbles are generated, charge transfer is not carried out properly so that the power generation property deteriorates. Furthermore, this part will also cause cell failure since decomposition of sensitizing dye or the like occurs.
Accordingly, a method to execute the sealing operation with resin has been proposed as one method to solve the above-described problems and to improve the durability (see Patent Document 1). Generally, resin has high gas permeability, so that the electrolyte is gradually leaked through inside the resin or the surface thereof. According to this proposal, an electrolyte storage portion is disposed at the upper part of the outside of the DSC for replenishing the electrolyte. Accordingly, since a solar cell is used by refilling the electrolyte from the electrolyte storage portion according to the quantity of the electrolyte that leaked out, a solar cell having an extended life-time thereof can be proposed.
However, in such a structure in which the electrolyte storage portion is disposed outside the DSC as the method described in Patent Document 1, the DSC will be large and bulky so that extra space is needed when the DSC is installed to be used, that is, it can not be easily handled. Moreover, since the method is to supply electrolyte from the electrolyte storage portion disposed in the upper part of the DSC by using gravity, the setting direction of the DSC was limited concerning a placement position of the electrolyte storage portion, and it was extremely difficult to effectively exhaust small-sized air bubbles generated around a central region of a cell.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-280085    [Non-Patent Document 1] O'Regan B, Gratzel M., “A low cost, high-efficiency solar cell based on dye-sensitized colloidal Ti02 films”, Nature 1991; 353: 737-739    [Non-Patent Document 2] M Spaeth re al., Prog. Photovolt: Res. Appl. 2003; 11: 207-220    [Non-Patent Document 3] R. Sastrawan re al., Sol. Ener. Mat. Sol. Cells 2006; 90: 11: 1680,    [Non-Patent Document 4] Patent Office: Collection of Standard Technologies, dye-sensitized solar cell, <http://www.jpo.go.jp/shiryou/s_sonota/hyoujun_gijyutsu/solar_cell/01_mokuji.htm>, Chapters 6-B-6-C